1. Field of the Invention
This invention relates generally to electron beam lithography (EBL), and more particularly to an EBL method for writing a curvilinear pattern over a large area with minimal accumulation of errors.
2. Description of the Related Art
EBL is a specialized technique for creating extremely fine patterns on a workpiece or specimen, such as a semiconductor wafer. In EBL, the specimen is covered with a resist film that is sensitive to the electrons and is moved across the electron beam (e-beam). The primary advantage of EBL is that it overcomes the diffraction limit of light and enables the patterning of features in the nanometer range. EBL has has yet to become a standard manufacturing technique because of its slow speed. Because the e-beam must be scanned across the surface to be patterned, typically raster-scanned in an x-y Cartesian coordinate system, the pattern generation is serial. This makes for very slow pattern generation compared with a parallel technique like conventional photolithography in which the entire surface of the specimen is patterned at once. As a result, EBL is used mainly to generate exposure masks to be used with conventional photolithography. For commercial applications, EBL is usually produced using dedicated e-beam tools or writing systems, such as those available from Leica Microsystems and Hitachi, Ltd.
Commercial e-beam writing systems use an x-y stage that moves the specimen in a Cartesian coordinate system in a plane orthogonal to the incident e-beam. The stage is divided into square fields in the x-y coordinate system and is moved in a raster technique from field to field in the x and y directions so that fields of the specimen are successively positioned under the e-beam. After a specific field has been positioned, the e-beam is scanned across subfields within that field to write the portion of the pattern within that field. These e-beam writing systems work well for their primary application, the patterning of semiconductor masks, wherein the entire specimen contains a large number of relatively small identical patterns corresponding to the individual semiconductor chips and the patterns contain a large number of straight lines. However, it becomes difficult to use these systems to write closed curvilinear patterns such as circles, and particularly circular patterns that extend over a large area of the entire specimen. This is because errors in movement of the stage from field to field accumulate so that the last portion of the circular pattern does not correlate with the first portion.
One application for e-beam writing of large-area circular patterns is for patterned magnetic recording disks. Magnetic recording hard disk drives with patterned magnetic recording disks have been proposed to increase data density. In a patterned disk, the magnetic recording layer on the disk is patterned into small isolated data islands arranged in concentric circular data tracks. Patterned disks also have nondata regions that are used for servo positioning of the read/write heads in the data tracks. To achieve patterned disks with areal data densities greater than about 300 Gbit/in2, the pattern period is typically below about 50 nm along-the-track and the diameter of the data islands is below about 30 nm. One proposed method for fabricating patterned disks with such extremely small features is by nanoimprinting with a master disk or “stamper” having a topographic surface pattern. In this method the magnetic recording disk substrate with a polymer film on its surface is pressed against the master disk. The polymer film receives the image of the master disk pattern and then becomes a mask for subsequent etching of the disk substrate. The magnetic layer and other layers needed for the magnetic recording disk are then deposited onto the etched disk substrate to form the patterned-media disk. The master disk for nanoimprinting can be fabricated by EBL provided the circular patterns can be written with high precision.
What is needed is an e-beam writing method for commercial Cartesian-type EBL systems that enables closed curvilinear patterns, in particular concentric circular patterns, to be written over relatively large areas without accumulation of errors caused by movement of the x-y stage from field to field.